This invention relates to lasers, and more particularly relates to fiber lasers.
Fiber lasers have been the subject of intense investigation due to their potential as a compact, inexpensive, and robust source of ultrashort laser pulses. Rare earth-doped fibers have been shown to be particularly advantageous for fiber laser designs in that they can provide, in a single element, the functionality of gain, self-phase modulation, and group-velocity dispersion. Mode-locked fiber laser designs incorporating rare earth-doped fibers have been shown for linear, ring, figure-eight, and reflector geometries.
Conventionally and historically, all-fiber mode-locked laser designs rely on operation in the soliton regime--that is, soliton pulse shaping is employed as a mechanism for producing laser pulses. This mechanism is based on the generation of solitons and their shaping by e.g., interference of two versions of the same pulse after appropriate modulation. The soliton pulse shape is maintained through the combined action of negative group velocity dispersion (GVD) and modelocking. Thus negative GVD is a requirement for effective soliton operation.
Kafka and Baer showed such a soliton fiber laser, Optics Letters 14, 1269 (1989) and U.S. Pat. No. 4,835,778, using an erbium-doped section of fiber to provide gain and a section of undoped fiber to ensure overall negative GVD. This laser uses a guided wave modulator for modelocking, allowing for bidirectional operation. While it is assumed that the erbium-doped fiber, 70 meters in length, generates laser emission in the negative-GVD region of the fiber, the exact contribution of the erbium-doped fiber was stated as unknown, and so 2 kilometers of standard telecommunications fiber was included to ensure negative GVD and correspondingly, soliton operation.
It has been later shown that this soliton operation leads to several undesirable phenomena; in perhaps the most limiting, soliton pulses are known to occur in chaotic bunches with no well-defined repetition rates--that is, there occur many pulses within one round trip time. In an effort to produce stable, uniformly periodic trains of soliton pulses, Hofer et al. and Fermann et al., Optics Letters 17, 807 (1992) and Optics Letters 18, 48 (1993) showed using a very short piece of rare earth-doped gain fiber, (neodymium and erbium, respectively) with a positive GVD, in combination with an intracavity prism pair (of inherently negative GVD) to obtain overall net negative GVD and soliton operation. A short piece of fiber was described as being advantageous for reducing the pulse reshaping that occurs per pass of the pulse through the fiber. Stable, bandwidth-limited soliton pulses as short as 180 fs and having an energy of 100 pJ were shown by Fermann to be produced using a 67 cm-long stretch of erbium-doped fiber, with positive GVD, and a polarizing ZnS prism pair, having negative GVD--passive amplitude modulation was achieved using nonlinear polarization evolution.
An all-fiber laser design that does not, like the lasers just described, rely on bulk intracavity elements for dispersion compensation has been shown superior in that unlike those lasers, it does not require "fine tuning" of the geometric relationship of the elements and is impervious to temperature-dependent instability of the element positions. Tamura et al. showed an all-fiber ring laser, Electronics Letters 28, 2226 (1992) that achieved great simplicity and robustness while at the same time achieving soliton operation. Here a 4.8 meter-long erbium-doped fiber, which provided negative GVD, was shown fully integrated with a nonlinear polarization rotator to achieve passive additive pulse mode-locking. It was demonstrated with this laser design, Optics Letters 18, 220 (1993), that unidirectional operation of a fiber ring with such a scheme provides further control of uniformly periodic single-pulse soliton operation and furthermore, allows for true self-starting (from noise alone).
All of these soliton-based laser designs face common limitations to achieving even higher energy than that demonstrated. Operation in the soliton regime constrains the single-pulse energy to be close to that of a fundamental soliton (E.sub.sol)--recall that a soliton is a pulse resulting from a balance between dispersive effects and self phase modulation whereby the pulse will propagate through a dispersive medium without changing shape. Thus a pulse that is too long will in effect make itself shorter until it reaches a balance point, E.sub.sol, and will then propagate at E.sub.sol. This fundamentally limits the achievable pulse energy and produces the phenomenon of random multiple-pulse trains described earlier. While using a shorter fiber length was shown to avoid multiple pulsing, the single pulse energy and duration remains limited.
Single pulse soliton energy is determined based on the soliton area theorem, which quantifies the relationship between the soliton pulse energy and width; for a given peak power, the duration and energy of a pulse are fixed. The peak power of a soliton laser may become clamped by, e.g., additive pulse modelocking, however, which may saturate because it is interferometric in nature. While in principle this lo saturation level is adjustable, as a practical matter it limits the ability of the laser to self-start, and so does not provide a practical alternative for increasing pulse energy.
A further pulse limitation is caused by the periodic perturbation of the soliton in its round trip around the laser fiber. The nonlinear phase shift in one round trip is determined by the pulse energy and width. Because this nonlinear phase shift is also limited by the sideband generation of the periodically perturbed Soliton, an upper limit to the energy of the soliton of a given width is imposed by the phase shift. A soliton becomes highly unstable when the period of perturbation approaches 8Z.sub.0, where Z.sub.0 is the soliton period. Because a mode-locked laser periodically perturbs the soliton pulse at the cavity round-trip length L, the shortest soliton that can be supported stably must have 8Z.sub.0 &gt;L. In practice, the shortest observed pulses typically have L.congruent.1Z.sub.0 or 2Z.sub.0. For ultrashort pulses, say &lt;100-fs, this creates a practical difficulty in that Z.sub.0 &lt;25 cm in standard fiber.
Thus, the substantial gains made in the design of all-fiber lasers have yet to achieve operation in the soliton regime that produces high-energy ultrashort laser pulses.